The intervertebral or spinal disc is an anatomically and functionally complex joint. The intervertebral disc is made up of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral end plates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
The spinal disc may be displaced or damaged due to trauma or disease. As a result of such displacement or damage, the nucleus pulposus may herniate and protrude into the vertebral canal or intervertebral foramen. Such a deformation is known as a herniated or “slipped” disc. This protrusion may press upon one or more of the spinal nerves exiting the vertebral canal through a partially obstructed foramen, thereby causing pain or paralysis in the area of its distribution. Similarly, spinal discs may degenerate with age or excessive use resulting in a decrease in disc height. One specific result of such disc height decline is the narrowing of the foramenal space, often causing pressure on the emanating nerve and causing pain and paralysis in the area of the nerve's influence. Pressure on the nerve and disc herniation often happen together.
Artificial intervertebral discs are used to replace damaged or diseased natural intervertebral discs. Various types of artificial intervertebral discs have been developed with the goal of restoring the normal kinematics and load-sharing properties of the natural intervertebral discs. Two such types are ball-and-socket joint type discs and the elastomer type discs.
We have described prosthetic intervertebral discs in application Ser. No. 10/903,276, titled “Prosthetic Intervertebral Disc and Methods for Using Same,” filed on Jul. 30, 2004, assigned to Spinal Kinetics Inc., the entirety of which is incorporated by reference.